Scaling Twitter: Making Twitter 10000 Percent Faster | High Scalability
There is some great information in here about scaling a RoR site - or any site for that matter. Earlier this year, I was in a position as the CTO to make a decision whether to migrate my site from Java to Ruby on Rails. I struggled a bit because I had heard of so many scaling problems. In the end (after a lot of testing), I convinced myself that there would be scaling problems, but most of the scaling problems would be due to decisions my team and I made. The problems would not all be with Ruby. Sure, Ruby runs a whole lot slower than Java, but that doesn't mean there are no Java apps out there that don't scale. If you architect your site correctly, more CPU cycles should not be a problem. More servers is always cheaper than a few extra weeks of developer time.

Bottom line is to spend your time designing your site to scale and don't sweat a few extra CPU cycles.